Defrost system and method for heat pumps

ABSTRACT

A system and method for defrosting an air source heat pump are presented. A defrost conduit extends from the discharge from a system compressor to the outdoor coil. The defrost conduit includes a normally closed defrost valve. When the need for defrost exists, the defrost valve is opened, and warm refrigerant is delivered from the compressor discharge to the outdoor coil to effect defrost of the outdoor coil.

FIELD OF THE INVENTION

This invention relates to the field of heat pumps. More particularly, this invention relates to the field of defrost technology for air source heat pumps.

BACKGROUND OF THE INVENTION

Air source heat pumps operate by extracting heat energy from outdoor air and delivering that heat energy to heat an interior space. In periods of high outdoor relative humidity combined with an outdoor temperature close to the freezing point of water, frost and/or ice will develop on the outdoor coil of the heat pump, thus impeding heat transfer to the coil surface areas. Defrost operation is required to remove the accumulated frost and/or ice. Defrosting may be accomplished on a timed schedule (time defrost), and/or by sensing the need to accomplish defrosting (demand defrost).

Whether defrosting is accomplished on a time basis or a demand basis, defrosting is typically accomplished by reverse cycle defrosting. In reverse cycle defrosting, a four-way valve is operated so that the heat pump is, in effect, operated in a cooling mode of operation wherein heat energy is extracted from the previously heated interior space to heat the refrigerant fluid circulating in the system, and the heat energy is transported to the outdoor coil to melt and remove the frost/ice that has accumulated on the outdoor coil. The interior of the previously heated space is thus impacted in two ways. One is by removal of significant amounts of heat energy that has just been delivered to the interior space. The other is by creating an uncomfortable flow of cool air felt by occupants of the previously heated space.

Another problem with reverse cycle defrosting is that the warm refrigerant is delivered to the top of the outdoor coil (the vapor outlet side in heating operation) and melts the frost/ice accumulation from the top of the coil downward. This can result in refreezing on the lower parts of the coil.

Reverse cycle defrosting impairs the efficiency of operation of heat pump systems, especially as the frequency of defrost operation increases, and it also results in discomfort of the occupants of the interior space to be heated.

SUMMARY OF THE INVENTION

The above-discussed problems of prior art heat pumps are overcome or alleviated by the present invention. In accordance with the present invention, an electrically operated flow control defrost valve is positioned downstream of the discharge of the compressor(s) of the heat exchange system, with the defrost valve being in a defrost refrigerant flow line connected between the compressor discharge and the bottom of the outdoor coil. The defrost valve is normally closed, so that refrigerant does not flow through the defrost line during normal (i.e. non-defrost operation) of the system. However, when defrost operation is desired, the defrost valve is opened, and warm refrigerant vapor is delivered from the discharge of the compressor(s) to the bottom of the outdoor coil to heat the outdoor coil and melt accumulated frost/ice. The defrost valve is closed when defrost operation is completed, and normal operation of the heat pump system is resumed.

An important feature of this invention is that it eliminates the need for the conventional reverse cycle defrosting heretofore used in heat pump systems. Accordingly, this invention eliminates the need to extract energy from the previously heated interior space, since warm indoor air is no longer used as the source of energy for defrosting the outside coil, and cold drafts, which discomfort occupants of the interior space, are also eliminated.

In the present invention, the frost/ice accumulation on the outdoor coil is melted from the bottom of the coil up to the top, thus eliminating or reducing the problem of possible refreezing of the coil when melted from the top down as in the prior art reverse cycle defrosting.

Instead of extracting energy from the heated interior space for defrost operation, the energy source for defrost operation in the present invention is a combination of the electrical input to the compressor(s) and an accumulator heating element in the heat pump system.

The present invention also incorporates a sensor to detect the build-up of frost/ice on the outdoor coil and initiate defrost operation when required. The sensor includes a light emitting diode (LED) and a photo-transistor secured between adjacent fins in the outdoor coil. A build-up of frost/ice between the LED and the photo-transistor partially blocks light transmission, and this blockage of light is used as a defrost initiating signal by the heat pump system microprocessor to initiate defrost operation for a predetermined time period, the time of defrost operation being a function of outdoor air temperature as sensed by the microprocessor at the initiation of defrost operation.

DESCRIPTION OF THE DRAWINGS

Referring to the drawings, where like elements are numbered alike in the several figures:

FIG. 1 is a schematic view of the system of the present invention in a heat pump system having heating and cooling capabilities.

FIG. 2 is a view showing the frost/ice accumulation detector of the present invention.

FIG. 3 is a schematic view of the system of the present invention in a heat only heat pump system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a heat pump system is shown which is capable of heating and cooling operation. The heat pump system preferably is a boosted air source system having a primary compressor 10 and a booster compressor 12 connected for operation in series. As shown, primary compressor 10 is operating, i.e., on, and booster compressor 12 is inoperative, i.e., off. However, it will be understood that both compressors can be operating, i.e., on, if desired. The system also has an outdoor coil 14 that functions as an evaporator in the heating mode of system operation to extract heat from the outside air, an indoor coil 16 that functions as a condenser in the heating mode of operation to deliver heat to a interior space to be heated, and a 4 way flow control valve 18 that functions to switch the system between the heating to the cooling modes of operation depending on the position of the 4 way valve. The solid line position of 4 way valve 18 is the heating position; the position shown in the dashed lines is the cooling position. Outdoor coil 14 can be, but need not be, an inverted “A” coil.

The system also has a refrigerant circulation conduit system including: a conduit segment 20 which receives compressed refrigerant from the discharge from primary compressor 10 and delivers the refrigerant through 4 way valve 18 as shown; a conduit segment 22 which delivers the compressed refrigerant to indoor coil 16 to heat the indoor space by heat transfer exchange with air (indicated by arrows flowing over the coil) supplied by a fan 16A; a conduit segment 24A that delivers the refrigerant through a check valve 24B and around a closed thermal expansion valve (TXV) 25 to a conduit segment 26; a conduit segment 26 including a thermal expansion valve 27 through which the refrigerant flows to a refrigerant distributor 14B and then to the bottom of outdoor coil 14 and through outdoor coil 14 to extract heat from the outdoor air flowing over coil 14 (indicated by arrows flowing over the coil) supplied by fan 14A; a conduit segment 28 that delivers the refrigerant through 4 way valve 18 as shown to conduit segment 29 and then to accumulator 30 where oil entrained in the refrigerant is separated for return to the compressor sumps (separation and return of the oil not shown); and the refrigerant is then delivered to conduit segments 31 and 32 and through check valve 34; and the refrigerant then flows to conduit segments 36 and 38 which receive the refrigerant from check valve 34 and deliver the refrigerant to the inlet to primary compressor 10. The flow of refrigerant through the conduit system for the heating mode of operation is indicated by the arrows in the various conduit segments.

If booster compressor 12 is also operating, check valve 34 is closed by the discharge pressure from booster compressor 12, and the refrigerant flows from branch conduit 31 to branch conduit 40, and then to the inlet to booster compressor 12; and the refrigerant discharged from booster compressor 12 is delivered via branch conduit 42 and branch conduit 38 to the inlet to primary compressor 10.

For operation of the system in the cooling mode, 4 way valve 18 is moved to the position shown in by the dashed lines, whereby the direction of flow of refrigerant in the system is reversed. In that case, the system operates as in a cooling mode. Heat is extracted from the indoor space at indoor coil 16, and the heated refrigerant is delivered through 4 way valve 18 and conduit segment 29 to accumulator 30 and to conduit segments 31, 32, 34, 36 and 38 to the inlet to primary compressor 10 if primary compressor 10 is on and booster compressor 12 is off (or if both compressors are on, the refrigerant from indoor coil 16 is delivered via conduit segment 22 and through 4 way valve 18 to conduit segment 29 and accumulator 30 and conduit segment 40 to the inlet to booster compressor 12, and then from the discharge from booster compressor 12 to the inlet to primary compressor 10). The compressed refrigerant discharged from primary compressor 10 then flows via conduit segment 20 and through 4 way valve 18 and via conduit segment 28 to the top of outdoor coil 14 and through coil 14 to discharge heat at the outdoor coil. The refrigerant then flows around closed TXV 27 via bypass line 26A and check valve 41 to conduit segments 26 and 24 to TXV 25 and to indoor coil 16 where heat is removed from the space to be cooled. The refrigerant is then delivered by conduit segment 22, 4 way valve 18, conduit segment 29, accumulator 30 and conduit segments 31, 32, 36 and 38 and check valve 34 to the inlet to primary compressor 10.

In prior art heat pump systems when operating in the heating mode, frost and/or ice can accumulate on the outdoor coil 14, necessitating the need to effect a defrost operation. That defrost operation typically involves reversing the operation of the system to the cooling mode, whereby warm refrigerant is delivered to flow from the top of outdoor coil 14 to the bottom of outdoor coil 14 to melt to frost and/or ice accumulated on the coil.

This is sometimes referred to as “reverse cycle defrost” operation. However, this reverse cycle defrost operation has several problems well known in the art. Perhaps foremost among these problems is that heat energy is extracted from the previously heated space by indoor coil 16 to heat the refrigerant flowing through the system, and that heat energy is transported to outdoor coil 14 to defrost the accumulated frost/ice on outdoor coil 14. The interior of the previously heated indoor space is thus negatively impacted in two ways. One is by removal of significant amounts of heat energy that has just been delivered to the interior space; the other is by creating an uncomfortable flow of cool air felt by occupants of the previously heated space. Another problem with the prior art reverse cycle defrosting is that the warm refrigerant is delivered to the top of the outdoor coil (the vapor outlet side in heating operation) and melts the frost/ice accumulation from the top of the coil downward. This can result in refreezing on the lower parts of the coil. Especially in situations of high humidity and a temperature at or near freezing for the outside air, frequent defrosting can be required, and system efficiency is impaired. These problems of the prior art are eliminated or substantially reduced in the present invention.

In accordance with the present invention, a defrost branch conduit 50 is connected from the discharge from primary compressor 10 to refrigerant distributor 14B at the bottom of outdoor coil 14. An electrically operated flow control defrost valve 52 in conduit 50 controls the flow of refrigerant in conduit 50 to outdoor coil 14. Valve 52 is a combination back pressure regulator/on-off control valve. When called upon to open, it will prevent the compressor discharge pressure from dropping below the equivalent of about 70 degrees F., thus ensuring that indoor coil 16 remains sufficiently pressurized to quickly resume heating upon completion of defrost. Defrost valve 52 is closed during normal heating operation of the heat pump system, so no refrigerant flows through conduit 50 to outdoor coil 14 during normal heating operation of the system. However, when frost and/or ice accumulates on outdoor coil 14 requiring defrost, a signal is delivered from the system microprocessor controller 54 to open defrost valve 52, whereby warm refrigerant vapor discharged from primary compressor 10 is delivered to distributor 14B and then to the bottom of outdoor coil 14 and flows through outdoor coil 14 to melt the accumulated ice and/or frost. The outlet from distributor 14B connects to each one of the circuits in outdoor coil 14 to evenly distribute the flowing refrigerant to the outdoor coil circuits. Distributor 14B has an internal orifice at its entry which further expands the refrigerant exiting TXV 27 during heating operation. Defrost conduit 50 is connected to distributor 14B downstream (in the direction of refrigerant flow during heating operation) of the internal orifice in distributor 14B so that the internal orifice does not restrict the flow of defrost refrigerant to outdoor coil 14. After passing through outdoor coil 14, the defrost refrigerant passes through conduit segment 28 and through 4 way valve 18 to conduit segment 29 and through accumulator 30 and is delivered to the inlet of primary compressor 10 (or to the inlet to booster compressor 12 if both compressors are on) for compression and delivery of warm refrigerant vapor through defrost conduit 50 to outdoor coil 14 to continue the defrost cycle.

Most importantly, defrost of outdoor coil 14 is accomplished without moving 4 way valve 18 to the cooling position, and without extracting heat energy from the interior space being heated (as happens in the prior art reverse cycle defrost operation), so the major problems of the prior art reverse cycle defrost operation are eliminated. Also, the warm refrigerant vapor delivered to outdoor coil 14 for defrost is delivered to the bottom of the coil (which previously received liquid refrigerant in the heating cycle), so the frost/ice on outdoor coil 14 is melted from the bottom to the top of the coil, thus reducing or eliminating the prior art problem of refreezing of the coil during defrost operation.

Accumulator 30 has a heating element 56 which is activated by controller 54 to provide additional heat input to the defrost refrigerant if the warm refrigerant vapor discharged from the compressor(s) is not sufficient to accomplish defrost operation. In effect, the electrical input to drive the primary compressor (or the electrical input to drive both compressors if both are operating) plus the electrical input to the heating element 56 in accumulator 30 is the energy source for defrost operation. It should be noted that signals from controller 54 to fans 14A and 16A stop the operation of those fans during defrost operation. Some prior art accumulators have included a heating element used to boil off liquid refrigerant in the accumulator to return the refrigerant to circulation in the system. In the present invention, the heating element in the accumulator performs the function of adding heat energy to the refrigerant to increase the heat energy content of the refrigerant vapor for defrost operation.

Referring now to FIG. 2, a detector system for detecting the need for demand defrost operation is shown. FIG. 2 shows a partial detail of the outdoor coil 14 of FIG. 1. Outdoor coil 14 has a series of refrigerant tubes 60, with heat exchange coil fins 62 mounted on the tubes 60. A defrost sensor 66 is mounted between a pair of adjacent fins to detect the accumulation of frost/ice on outdoor coil 14 and trigger operation of the defrost cycle. Defrost sensor 66 includes a light emitting diode (LED) 68 and a phototransistor 70 for sensing the light emitted by LED 68. Prior to the accumulation of frost/ice sufficient to require defrost operation, enough of the light emitted by LED 68 is sensed by phototransistor 70, and phototransistor 70 delivers a signal to microprocessor 54 to keep defrost valve 52 closed However, when an amount of frost/ice accumulates on outdoor coil 14 sufficient to require defrost operation, the light received at phototransistor 70 drops below a threshold level, and the signal from the phototransistor 70 to microprocessor 54 also drops below a threshold level. When this happens, microprocessor 54 delivers a signal to defrost valve 52 to open valve 52 and initiates defrost operation. Microprocessor 54 can be programmed either to terminate defrost operation after a predetermined period of time, or to continue defrost operation until the light received at phototransistor 70 from LED 68 is sufficient to indicate that defrost operation has been completed, at which time microprocessor 54 closes defrost valve 52 to terminate defrost operation. A similar detector for ice build-up in aircraft carburetors is the “Iceman” probe of Lamar Technologies Corporation.

With the use of the detector system of FIG. 2, defrost operation is initiated early in the frost/ice accumulation process. Accordingly, the deposited frost/ice is removed quickly from the outdoor coil before any significant impeding of heat transfer from outdoor air into the coil is encountered.

Operation of the defrost system of the present invention can, if desired, be modulated by sensing the humidity and temperature of the outdoor air to adjust the operation of the defrost system for conditions of high humidity and low temperature

Referring now to FIG. 3, an embodiment of the present invention is shown for a “heating only” system, i.e., a system which has the capacity to heat, but does not have the capacity to cool. The components of the system of FIG. 3 are numbered as in FIG. 1. “Heating only systems are known in the prior art. However, even though such systems do not need a 4 way valve to switch to the cooling mode of operation, most prior art “heating only” systems still require the presence of a 4 way valve to reverse the flow of refrigerant to accomplish reverse cycle defrosting. However, with the presence of defrost conduit 50 and defrost valve 52 to deliver warm refrigerant vapor to defrost outdoor coil 14, a 4 way valve is not required for the heating only system of FIG. 3.

While preferred embodiments of the present invention have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described and shown by way of illustration and not limitation. 

1. A heat pump system including: at least one compressor; an indoor coil for delivering heat energy to an indoor space to be heated; an outdoor coil for extracting heat energy from outside air to be used to heat the indoor space; a conduit system connected between said one compressor and said indoor and outdoor coils for circulating refrigerant from said one compressor to said indoor coil and said outdoor coil and from said outdoor coil back to said one compressor; a defrost conduit connected between said one compressor and said outdoor coil; and a defrost flow control valve in said defrost conduit, said defrost control valve being normally closed, and said defrost control valve being opened to deliver refrigerant through said defrost conduit from said one compressor to said outdoor coil to defrost said outdoor coil.
 2. A heat pump system as in claim 1, further comprising: a detector in said outdoor coil, said detector detecting accumulation of ice on said outdoor coil and generating a defrost signal calling for defrost operation; a controller, said controller receiving said defrost signal from said detector and generating a signal to open said defrost valve to initiate defrost operation.
 3. A heat pump system as in claim 2 wherein said detector comprises: a light emitting diode; a phototransistor positioned to receive light from said light emitting diode and generating said defrost signal to said controller as a function of intensity of light from said light emitting diode sensed by said phototransistor.
 4. A heat pump system as in claim 1 wherein; said defrost flow control valve is a combination back pressure and on-off flow control valve.
 5. A heat pump system as in claim 4 wherein; said defrost flow control valve prevents discharge pressure from said one compressor from dropping below a predetermined equivalent temperature level.
 6. A heat pump system as in claim 1, further comprising: said defrost conduit is connected to deliver refrigerant to a bottom of said outdoor coil during defrost operation to defrost said outdoor coil from the bottom toward a top thereof.
 7. A heat pump system as in claim 1, further comprising: an accumulator connected in said conduit system between said outdoor coil and said one compressor; and a heating element in said accumulator, said heating element being activated during defrost operation to increase heat energy of said refrigerant delivered to said outdoor coil to defrost said outdoor coil.
 8. A heat pump system comprising: at least one compressor; an outdoor coil for extracting heat energy from outside air to be used to heat an indoor space; an indoor coil for delivering heat energy to an indoor space to be heated; a conduit system for circulating refrigerant fluid from said one compressor to said indoor coil and said outdoor coil and back to said one compressor; a 4 way valve in said conduit system for reversing flow of refrigerant in said conduit system to change a mode of operation of the heat pump system from heating operation to cooling operation; a defrost conduit connected between said one compressor and said outdoor coil; and a defrost flow control valve in said defrost conduit, said defrost control valve being normally closed, and said defrost control valve being opened to deliver refrigerant through said defrost conduit from said one compressor to said outdoor coil to defrost said outdoor coil.
 9. The heat pump system as in claim 8 wherein; refrigerant is delivered to said outdoor coil to effect defrost operation without operation of said 4 way valve.
 10. A heat pump system as in claim 8, further comprising: a detector in said outdoor coil, said detector detecting accumulation of ice on said outdoor coil and generating a defrost signal calling for defrost operation; a controller, said controller receiving said defrost signal from said detector and generating a signal to open said defrost valve to initiate defrost operation.
 11. A heat pump system as in claim 10 wherein said detector comprises: a light emitting diode; a phototransistor positioned to receive light from said light emitting diode and generating said defrost signal to said controller as a function of intensity of light from said light emitting diode sensed by said phototransistor.
 12. A heat pump system as in claim 8 wherein; said defrost flow control valve is a combination back pressure and on-off flow control valve.
 13. A heat pump system as in claim 12 wherein; said defrost flow control valve prevents discharge pressure from said one compressor from dropping below a predetermined equivalent temperature level.
 14. A heat pump system as in claim 8 wherein: said defrost conduit is connected to deliver refrigerant to a bottom of said outdoor coil during defrost operation to defrost said outdoor coil from the bottom toward a top thereof.
 15. A heat pump system as in claim 8, further comprising: an accumulator connected in said conduit system between said outdoor coil and said one compressor; and a heating element in said accumulator, said heating element being activated during defrost operation to increase heat energy of said refrigerant delivered to said outdoor coil to defrost said outdoor coil.
 16. The method of effecting defrost operation in a heat pump system, comprising: operating at least one compressor to circulate a refrigerant to a first coil to deliver heat energy to an interior space to be heated, and to a second coil to extract heat energy from outside air; connecting a defrost conduit between said one compressor and said outdoor coil; positioning a defrost flow control valve in said defrost conduit, said defrost control valve being normally closed; and opening said defrost control valve to deliver refrigerant through said defrost conduit from said one compressor to said outdoor coil to defrost said outdoor coil.
 17. The method of effecting defrost operation as in claim 16, further comprising: positioning a detector in said outdoor coil to detect accumulation of ice on said outdoor coil and generate a signal calling for defrost operation; delivering said defrost signal to a controller, said controller receiving said defrost signal from said detector and generating a defrost signal to open said defrost valve to initiate defrost operation.
 18. The method of effecting defrost operation as in claim 17 wherein the positioning a detector in said outside coil comprises: positioning a light emitting diode in said outdoor coil; and positioning a phototransistor in said outdoor coil to receive light from said light emitting diode and generating said defrost signal to said controller as a function of intensity of light from said light emitting diode sensed by said phototransistor.
 19. The method of effecting defrost operation as in claim 16, further comprising: connecting said defrost conduit to deliver refrigerant to a bottom of said outdoor coil during defrost operation to defrost said outdoor coil from the bottom toward a top thereof.
 20. The method of effecting defrost operation as in claim 16 wherein; said defrost flow control valve is a combination back pressure and on-off flow control valve to prevent discharge pressure from said one compressor from dropping below a predetermined equivalent temperature level.
 21. The method of effecting defrost operation as in claim 16, further comprising: positioning a heating element between said outdoor coil and said one compressor; and activating said heating element during defrost operation to increase heat energy delivered to said outdoor coil to defrost said outdoor coil. 